This application is based upon and claims the benefit of priority from the prior Japanese Patent Application No. 11-143465, filed May 24, 1999, the entire contents of which are incorporated herein by reference.
The present invention relates to a radio communication system that is applied to, for example, indoor wireless LAN systems and adapted for data transmissions between a base station and a number of terminal stations.
Radio communications in indoor space are highly susceptible to fading due to multipath interference (multiple reflected waves). It is therefore effective to transmit and receive data with directivity concentrated in the direction of maximum reception. For example, Japanese Unexamined Patent Publication No. 9-232848 discloses radio communications equipment that is constructed, as shown in FIG. 19, from an array antenna 1, circulators 2, receiving modules 3 each comprising a low-noise amplifier and a down converter, analog-to-digital (A/D) converters 4, quasi-coherent detectors 5, local oscillators 6 and 7, a digital beam former (DBF) 8, a beam selector 9, a maximum ratio combiner 10, a demodulator 11, an in-phase divider 12, phase/amplitude correction units 13, a weight separation calculation circuit 14, a transmission weight calculation circuit 15, quadrature modulators 16, transmitting modules 17 each consisting of an up converter and a transmitting power amplifier, and transmitting local oscillators 18 and 19.
Signals received by the antenna elements of the array antenna 1 are amplified and converted into digital signals in the A/D converters 4. The digital signals are subjected to quasi-coherent detection in the quasi-coherent detectors 5 and then combined in the maximum ratio combiner 10 with their weight values varied so that the S/N ratio at receive time becomes maximized. At transmit time, the transmitting weight value calculation circuit 15 calculates transmitting weight values on the basis of the receiving weight values calculated by the weight separation/calculation circuit 14. After transmit signals are weighted by the transmitting weight values, a transmit main beam is formed and transmitted in the direction of maximum reception from the array antenna 1 via the quadrature modulators 16 and the transmitting modules 17.
The conventional equipment needs to calculate the direction of arriving radiation in real time (namely, in sequence) and switch the directivity in order to track the station with which the connection has been set up. As a result, calculation processing is performed mainly by hardware rather than by software. Thus, the equipment has a problem of being complex in circuit arrangement and increasing in size.
To solve this problem, Japanese Unexamined Patent Publication No. 9-219615 discloses an adaptive array transmitter-receiver, which is constructed, as shown in FIG. 20, from an antenna array 21 comprised of a number of antenna elements, weighting units 22 for weighting the amplitude and phase of each of signals which are to be transmitted from or are received by the antenna elements, a divider/combiner 23 for distributing transmit signals to the antenna elements or combining received signals from the antenna elements through the weighting units 22, an interface 25, and an external operations unit 26. The external operations unit 26 can perform calculations for adaptive control of the antenna directivity in non-real time. The weight factor switching from terminal to terminal is made on a time division basis with a time slot preallocated for each terminal.
The weighting units 22 varies each of the phase and amplitude of each of signals output from the divider/combiner 23 by a predetermined amount or multiplies each of the signals by a complex weight value. Electromagnetic waves are then emitted from the respective antenna elements 21 in a desired transmitting radiation pattern. On the other hand, received signals from the antenna elements 21 are controlled in phase and amplitude or multiplied by complex weight values in the weighting units 22 and then combined in the divider/combiner 23. In this manner, a desired receiving radiation pattern is formed.
However, in the adaptive array transmitter-receiver disclosed in Japanese Unexamined Patent Publication No. 9-219615, recalculations of the weight values are performed in the external operations unit 26 on the results of reception of reference signals transmitted from the terminal stations at relatively long intervals of time. Another way involves recalculating weight values in the external operations unit 26 on the basis of weight values used in the last time slot and using the recalculated weight values as weight values in the current time slot. In this manner, variations in the electromagnetic wave propagation environment are coped with.
The recalculations of weight values at long intervals of time cannot exactly follow variations in the propagation environment because variations in the electromagnetic wave propagation environment do not occur always at regular intervals.
In the latter method, if the time interval between each time slot is reduced, variations in the electromagnetic wave propagation environment will be coped with. However, the calculations of weight values at short time intervals will require an expensive unit adapted for fast operations. In this case, the calculations will result wastefully when the propagation environment suffers little from variations.
Thus, trying to detect variations in the electromagnetic wave propagation environment at regular intervals will result in failure to catch variations in the environment fully and in redundant, inefficient calculations of weight values.
It is therefore an object of the present invention to provide a radio communication system which allows good communications at all times irrespective of variations in electromagnetic radiation propagation environment and allows the hardware arrangement to be made simple.
According to the present invention, there is provided a radio communication system including a base station having a receiving section which weights the amplitude and phase of each of received signals from a plurality of antenna elements by amplitude and phase weight values set in the receiving section, then combines the weighted signals into a composite signal and demodulates the resultant composite signal and a transmitting section which divides a modulated signal into a plurality of transmit signals, then weights the amplitude and phase of each of the transmit signals by amplitude and phase weight values set in the transmitting section and radiates each of the weighted transmit signals from a corresponding respective one of the antenna elements, and a number of terminal stations each of which is adapted to perform radio communications with the base station, the base station including: receiving condition detecting means for detecting a variation in the receiving condition of the receiving section; and weight value rewrite means for, when a variation in the receiving condition of the receiving section is detected by the receiving condition detecting means, recalculating amplitude and phase weight values to be used in the receiving and transmitting sections and rewriting the amplitude and phase weight values already set in the receiving and transmitting sections by the recalculated weight values.
In an aspect of the present invention, the receiving condition detecting means includes received signal level detecting means for detecting a lowering of the received signal level in the receiving section, and the weight value rewrite means recalculates and rewrites the weight values in the receiving and transmitting sections when a lowering of the received signal level is detected by the received signal level detecting means.
In another aspect of the present invention, radio communications are performed between the base station and each of the terminals station by a frequency hopping spectrum spreading technique, the receiving condition detecting means includes error rate detecting means for detecting an increase in error rate of a demodulated signal by the receiving section, and the weight value rewrite means recalculates and rewrites the weight values in the receiving and transmitting sections when an increase in error rate is detected by the error rate detecting means.
In still another aspect of the present invention, radio communications are performed between the base station and each of the terminals station by a direct spread spectrum spreading technique, the receiving condition detecting means includes communication disabled state detecting means for detecting a communications disabled state of the base station, and the weight value rewrite means recalculates and rewrites the weight values in the receiving and transmitting sections when a communications disabled state of the base station is detected by the communications disabled state detecting means.
Additional objects and advantages of the invention will be set forth in the description which follows, and in part will be obvious from the description, or may be learned by practice of the invention. The objects and advantages of the invention may be realized and obtained by means of the instrumentalities and combinations particularly pointed out hereinafter.